1. Field of the Invention
This invention relates to a ring generator circuit that produces a ring output for a plurality of telephones simultaneously. More specifically, this invention relates to an on-board ring signal generator for use, for example, on fiber optic lines.
2. Description of the Related Art
Conventional ring generator circuits are typically large units located in a central telephone switching office (central office). In such installations, one ring generator circuit must drive many telephone lines at the same time. A line for each of the telephones is connected to the central office where the ring generator is located. Often, the telephone traffic results in a number of telephones ringing at the same time. Therefore, the ring generator circuit must be designed to drive many lines and ring many telephones simultaneously. The exact requirements of such ring generator circuits are based on traffic statistics of the number and frequency of telephone calls received in an office.
Increasing numbers of telephone users and the development of alternative communication and transmission techniques, such as fiber optic communications, have created new problems and requirements for ring generator circuits. For example, FIG. 1 shows a system in which the central office 1 sends information through ISDN network 3 over digital line 5. That information is received by ISDN network 7, in which telephones 10 connected to local lines 9 are rung. This system is in contrast to the more traditional system shown in FIG. 2, in which central office 1 rings each of lines 9 individually, even if the lines are on the same trunk.
Referring back to FIG. 1, in the case where digital line 5 is, for example, a fiber optic line, the line cannot carry the high power ring signal used in the traditional systems of FIG. 2. In the United States, such ring signals are specified at a DC level of -48 volts with an AC signal of 86 volts RMS. In view of the characteristics of the transmission media, such as the limitations of fiber optic cables, the need to provide local ring generators has become critical.
In one example, a local ring generator manufactured by SGS Thompson is expensive and produces only 60 volts RMS and 30 volts DC. Therefore, the SGS Thompson on-chip regulator cannot easily meet the current U.S. specifications for supplying ringing voltages for a plurality of telephone lines. Consequently, there is a need for a new local ring generator for ringing a plurality of lines and which can accommodate the current voltage specifications for both ringing and non-ringing conditions.